1. Field of the Invention
The present invention is generally directed to communication systems and networks and is particularly directed to such systems and networks which use multi-carrier protocols such as orthogonal frequency division multiplexing and discrete multi-tone protocols, and to techniques for communicating thereover.
2. Background of Related Art
Orthogonal frequency division multiplexing (OFDM) and discrete multi-tone (DMT) are two closely related formats which have become popular as communication protocols. Systems of this type take a relatively wide bandwidth communication channel and break it into many smaller frequency sub-channels. The narrower sub-channels are then used simultaneously to transmit data at a high rate. These techniques have advantages when the communication channel has multi-path or narrow band interference.
The following discussion of the prior art and the invention will address OFDM systems; however, it will be understood that the invention is equally applicable to DMT systems (as well as other types of communication systems) with only minor modifications that will be readily apparent to those skilled in the art.
A functional block diagram of a typical OFDM transmitter is shown in FIG. 1. Here, an incoming stream 10 of N symbols d0, d1 . . . dN-1 is mapped by a serial-to-parallel converter 20 over N parallel lines 30, each line corresponding to a particular subcarrier within the overall OFDM channel. An Inverse Fast Fourier Transform (iFFT) processor 40 accepts these as frequency domain components and generates a set 50 of time domain subcarriers corresponding thereto. Each set of time domain subcarriers is considered a symbol. The rate at which these symbols are created determines the rate at which transitions are made on each of the individual carriers (one transmission per symbol time). The time domain subcarriers are converted by a parallel-to-serial converter 60. Due to the characteristics of the inverse Fourier transform, although the frequency spectra of the subcarrier channels overlap, each subcarrier is orthogonal to the others. Thus, the frequency at which each subcarrier in the received signal is evaluated is one at which the contribution from all other signals is zero.
A functional block diagram of the corresponding OFDM receiver is shown in FIG. 2. Here, an OFDM signal is received and converted into multiple time domain signals 210 by a serial-to-parallel converter 220. These signals are processed by a Fast Fourier Transform (FFT) processor 230 before being multiplexed by parallel-to-serial converter 240 to recover the original data stream 250.
FIG. 3 shows a plot of the transmitted frequency spectrum from an OFDM system. The number of carriers within the signal is determined by the size of the iFFT processor in the transmitter and corresponding size of the FFT processor in the receiver. The spacing of the individual carriers within the signal is dependent on the rate at which the iFFT symbols are generated (the symbol rate). This is generally proportional to the rate at which the iFFT and FFT processors are being clocked. Finally, the overall bandwidth occupied by the signal is roughly equivalent to the number of carriers multiplied by the carrier spacing.
The symbol rate is generally chosen to limit the effect of multi-path interference in the channel. When the rate of iFFT/FFT symbol generation is low, the rate of the symbols going over the channel is slow, and the carrier spacing is close. These slow symbols are long in time, much longer than the longest echoes within the multi-path delays of the channel. Therefore, it is possible to avoid or minimize the multi-path echoes, since they are much shorter than the data symbols themselves.
In some multi-carrier systems, the amount of power allocated to each carrier is varied according to the quality of the channel over which the signal will be sent. In addition, the complexity of the modulation constellation is also varied according to the channel on a per carrier basis. For example, some carriers may use 4-QAM modulation, while others use 16-QAM, 64-QAM or even more complex modulation. The more complex modulations allow more data to be transmitted in a single symbol or period of time. However, they require a much better signal to noise ratio in order to operate correctly. In other systems, it may be difficult to determine details about the channel, or the channel may change rapidly in time, such that this adaptation of the multi-carrier transmission is not practical. Rapidly changing channel conditions are common in radio communications.
Although some existing multi-carrier systems adapt the power allocation and modulation complexity as described above, existing multi-carrier systems maintain a constant number of carriers (constant size of the iFFT and FFT processors) and a constant carrier spacing (constant rate of iFFT/FFT symbol generation), and therefore a constant overall occupied bandwidth. The constant carrier spacing is chosen to insure that multi-path echoes are a small portion of the data symbol time in all possible channels that the communication system might encounter.
It is advantageous to minimize the number of carriers in use. The number of carriers is directly related to the size of the iFFT processor in the transmitter and corresponding FFT processor in the receiver. The complexity and power consumption of an iFFT or FFT processor increases as N*log(N), where N is the size of the processor, and therefore the number of carriers present in the signal. To limit complexity and particularly power consumption, it is therefore desirable to minimize the number of carriers in use. Additionally, it is desirable to generate the iFFT/FFT symbols at the highest rate possible. This increases the symbol rate, and thereby increases the data rate within the channel. Taken together, the goal of low complexity, low power, and high data rate pushes toward a system with few carriers and a high iFFT/FFT symbol generation rate. However, there is a limitation. As the symbol rate becomes higher, the symbols become shorter in time. For a given channel, the multi-path echoes will become a larger fraction of the symbol time, and will increasingly corrupt the communication. In addition, since the total bandwidth occupied is roughly equal to the number of carriers times the carrier spacing (proportional to the symbol rate), the overall occupied bandwidth may also increase as the symbol rate is increased.
Existing multi-carrier systems, which maintain a fixed number of carriers, a fixed symbol rate, and a fixed overall bandwidth, do not operate under optimal conditions. Because these fixed parameters must be chosen to accommodate the worst possible channel conditions, they are often far too conservative and not optimal for the channel currently available.